What is the exact means of gap in research paper?

Thanks for your good question! 

Research is a tool to fill the knowledge gaps in the particular area or field. We conduct research to identify/investigate/explore/improve/discover something. Research is considered as a systematic approach to achieving the goal, which is contribution/knowledge. We add or contribute knowledge or fill the gaps in knowledge through research.

If we ask about the research gap, then we usually answer that this has done or not yet done. Ultimately it is the Knowledge.